One of the great things about the new 1st Time Homebuyer tax credit is that it can be claimed as soon as you close on your home.
If you have already filed can file an amended 2008 tax return with a 1040X form. This can be done even if a home was purchased early in 2009 and the original tax return claimed the $7,500 credit from last year.
Specifically home buyers use IRS Form 5405 to determine their tax credit amount, and then claim this amount on Line 69 of their 1040 income tax return. You will want to be sure that you qualify under the limits of the 1st Time Homebuyer tests.